Freshly baked tarts -- in their infinite variations -- are a natural showcase for the harbingers of spring: plump, juicy strawberries; thin, young asparagus; and small, tender artichoke hearts.

Tarts can be sweet or savory; round, square, or rectangular; delicate or hearty; and they are equally suited to being served as desserts, hors d'oeuvres, snacks or as the main course. Besides fruits and vegetables, the fillings can include custards -- sweet or not; or a dessert tart can be concocted from jams. The pastry used is almost as varied; it may be cakelike, cookie-style or flaky.

Many cooks think of tarts simply as open-faced pies, but for the most part tarts suggest a delicacy that pies rarely achieve. In Europe, a tart is often called a flan because of the metal flan ring traditionally used to shape and bake this pastry.

Though tarts and flans have been consumed since at least the sixth century, they became more popular during the Middle Ages, when plates were scarce, according to the "Dictionary of Gastronomy" by Andre Simon and Robin Howe (The Overlook Press, $37.50, 1978). Ingenious cooks from this period used bread crusts and dinner rolls as edible holders for their main dishes. Once the entree was consumed, so too, was the bready underliner.

In time, the crust was refined to mixtures more like cake or pastry. The fillings also became slightly daintier and the garnishes were often artfully arranged. Depending on their size, they came to be called tarts or tartlets (from the French word tarte, meaning pie).

Centuries later, tarts still consist of two main parts: the filling and the crust. Fillings usually are pretty straightforward. Their preparation rarely requires any particular expertise or training. Crusts are another matter. They require a bit of practice and may take a little time to prepare properly. And the variety of crusts for tarts is almost as wide-ranging as the choice of fillings.

Nicholis Malgieri, a master patissier who teaches baking techniques at the Peter Kump School in New York, is well versed in the art of making pastries and tarts. His extensive training includes a diploma from the Culinary Institute in Hyde Park, N.Y., an apprenticeship in Switzerland, and he has served as pastry chef for the Hotel de Paris in Monte Carlo and Windows of the World in New York City. In his classes, Malgieri teaches a number of different tart crusts.

"I usually teach three different types of dough for tart crusts," says Malgieri. "The first is a pa te brise'e, or a classic flaky pastry made with two parts flour, salt, three parts butter, and some water. It makes a very flaky, buttery pastry. I also like to add a tiny bit of sugar for color and taste. For savory tarts, I add a little baking powder to lighten it. For beginner students, I recommend adding a little egg to the dough; it makes it easier to roll out.

"For the second dough, or pa te sucre'e, which is a sweet flaky pastry, I increase the quantity of sugar, add some baking powder, which lightens the dough and causes it to rise away from the pan while baking, eliminating the need to prebake the crust before filling it. I use this dough for sweet tarts with a cooked filling; the filling and the crust bake at the same time.

"The third, and most impressive dough," says Malgieri, "is pa te feuillete'e or puff pastry. This crust is prepared in a lengthy process of rolling and folding in a large quantity of butter into a flour-and-water dough. The resulting pastry is multi-layered, flaky, and light as a feather. I prefer this type of dough in most professional situations because it looks so impressive and elegant. And I will use it much the same way I do with a pa te brise'e, baking it and then garnishing it with combinations of ripe, fresh fruit."

Like Malgieri, Flo Braker, author of "The Simple Art of Perfect Baking" (William Morrow and Company, $24.95, 1984), is no stranger to tartmaking. In the past 10 years she has been teaching baking all over the country and studying with master pastry chefs in the U.S., France and Switzerland, not only perfecting her art, but learning through trail-and-error some of the potential trouble spots. In a telephone interview from her California home, she elaborated on some common trouble areas and made suggestions for their remedy:

* "The fruit of filling, in my opinion, should be the star of the tart. In selecting the type of crust, you should keep this in mind. You should also think of taste, texture and appearance. The crust should highlight and accentuate the flavor of the filling."

* "Of course, you make a dough by hand, in a mixer, or in a food processor, and although I am a fan of the latter two methods, I feel that it is important to first start out making crusts by hand. Success in baking relies on your senses, even common sense. In pastrymaking, I suggest you work with your hands to keep in contact with the ingredients and to give you a sense of what is happening. (Is the butter too soft; is the dough too moist?) After becoming accustomed to the feel and appearance of making the dough, then you can transfer the process to a machine."

* "The common tendancy with sweet doughs is to roll the crust too thick. As a result, it does not bake all the way through and it becomes soggy. Roll the dough no thicker than 1/8 inch."

* "To roll out sweetened dough evenly, cut and glue strips of cardboard to form an 1/8-inch thickness. Place the dough between these strips and roll out until the dough is level with the top surface of the cardboard strips."

* "Fillings are primarily thickened to prevent sogginess. This is accomplished with a variety of starches. Acidity in the filling and freezing will break down the filling, making it runny."

* "To prevent juicy or thin fillings from soaking into the crust and making it soggy, brush a thin film of unsalted butter or melted chocolate on the baked crust to prevent sogginess. Since fat is not soluable, the butter or chocolate will not dissove as would an apricot glaze or a washing of egg white."

Braker also believes that it is crucial to be organized when baking. This initial pre-organization, she contends, demystifies the baking process.

There are a number of different types of pans used for making tarts. One of the most traditional is the round, fluted flan ring, coupled with a heavy baking sheet. As the crust bakes, the pastry moves away from the ring so that it may be removed before serving. Most French tartmakers fancy tinned steel fluted pans with removable bottoms for sweet tarts, and white porcelain quiche pans for savory renditions. The porcelain pans are attractive enough that the tart may be served directly from the pan. All of these pans are available at any well-stocked specialty cookware store.

The filling usually defines the tone of the tart -- declaring it sweet, savory, dainty or hearty. For sweet fresh fruit fillings, there are the raw, fresh fruit varieties -- with fresh strawberries, raspberries, bananas, grapes and kiwis. Cooked fruit and vegetable garnishes are made with poached pears, apples, apricots, asparagus, leeks and many others. And then there are the sweet and savory custard flans, as well as the more delicate confections, garnished with pastry cream.

This spring, why not celebrate the bounty of fresh new vegetables and fruits by baking a spring tart? An assortment of spring tart recipes follows: FLO BRAKER'S CLASSIC SWEET PASTRY (Makes three 8-inch tart crusts) 1/2 pound (2 sticks) butter 2 1/2 cups unsifted all-purpose flour 1/8 teaspoon salt 1/2 cup sugar 1 large egg 1 teaspoon vanilla extract

Remove the butter from the refrigerator about 30 to 40 minutes before preparing pastry. Stick a finger into it to feel if it is pliable and close to room temperature, about 65 to 70 degrees.

Pour the dry ingredients onto a smooth work surface. If it's a very warm day, a cold work surface, such as a marble slab or a chilled plastic cutting board, is helpful. With the side and fingers of your hand, clear an 8-inch circle out of the center of the dry ingredients, creating a well into which you will place other ingredients. Mix the egg and vanilla in a small bowl just to combine and place in well along with butter, cut into pieces.

Using the tips of your thumb and first two fingers, rub the egg-butter mixture together until a thick paste is formed. Still using your fingers, gradually pull in the wall of flour by drawing it into the center and mixing it with the paste. The mixture will be sticky to begin with, but as more flour is added, it will become malleable and uniform. Continue adding the dry ingredients until a dough is formed.

With the heel of your hand, smear a small amount of the dough (the size of an egg) on your work surface by pushing it away from you. Repeat with small amounts of the remaining dough. When you've worked all the dough in this manner, give it a couple more strokes to bring it together. Not only is this manipulation necessary for forming the dough into a cohesive unit, but it's important for developing enough structure to make rolling and working with it more manageable. Divide the dough in thirds, flatten each third into a round disk, 4 to 5 inches each, and wrap in plastic. Refrigerate for 1 hour, or until the dough is slightly chilled and slightly firm. Use as directed. FRESH STRAWBERRY TART (6 to 8 servings) 1/3 recipe Flo Braker's Classic Sweet Pastry, prepared as directed Flour for dusting 1 tablespoon unsalted butter, softened to room temperature 1 1/2 quarts strawberries, rinsed and hulled 1/4 cup apricot jam 1/4 cup Grand Marnier

If the dough is cold and firm, remove it from the refrigerator to room temperature for 1 hour, or until it is still cool and feels malleable. Dust the work surface lightly with flour and position the rolling pin across the center of the disk. Push the pin away from you in one stroke; use just enough pressure to extend the dough about 2 inches. Let up on the pressure near the edge. If any tearing or cracking occurs as you roll, press the edges together, sprinkle with a little flour, and continue to roll.

Gently lift dough, and rotate it one-eighth turn. Repeat rolling procedure, working from the center out, always rotating the dough a one-eighth turn in the same direction. Lightly flour the surface when necessary to prevent sticking. Continue rolling and rotating until the circle of dough is 1/8-inch thick and measures about 14 inches in diameter. Lift the dough up with the rolling pin and fit into a 10-inch quiche pan or a flan ring placed on a cookie sheet, allowing a 1-inch overhang. Crimp the edges decoratively and refrigerate 20 minutes.

Position the rack in the lower third of the oven. Bake the pastry at 350 degrees for 17 to 22 minutes, or until the shell appears golden and contracts from the side. After an initial 5 to 7 minutes in the oven, the shell may blister; if so, prick the bottom in three or four places with a metal skewer, allowing the steam to escape so that the dough will fit snugly in the pan. Remove and cool completely.

With a pastry brush, brush a very thin layer of the softened butter over the crust. Arrange the strawberries, pointed end up, one next to the other, to line the crust. Combine the apricot jam and the Grand Marnier in a saucepan and heat until boiling. Remove and cool slightly, then brush the strawberries until they glisten with the mixture. Let sit for 10 minutes and serve. LEMON SUNSHINE TART (10 servings) $ 1/3 receipe Flo Braker's Classic Sweet Pastry FOR THE FILLING£ 3 large eggs 1/2 cup sugar Grated rind and juice of 2 lemons, plus 1 lemon, thinly sliced$$1/2 cup melted unsalted butter, cooled to room temperature 1/2 cup sugar 1/2 cup water

Prepare the crust as directed in the recipe above. Roll out the crust as directed in the Fresh Strawberry Tart recipe and line a 10-inch tart or flan pan with the dough. Crimp the edges decoratively and chill briefly.

Line the crust with parchment paper or aluminum foil and fill with beans or rice. Bake at 400 degrees about 10 minutes, until the edges are beginning to brown. Remove the paper and the beans or rice and remove the tart. Reduce the oven to 375 degrees.

In a mixing bowl, beat the eggs with the sugar until thick and doubled in volume. Add the lemon rind and juice, and continue beating to incorporate the flavoring. Add the melted butter and beat lightly. Pour the mixture into the prebaked tart shell and place the pan towards the bottom of the oven. Bake for 25 minutes, or until the lemon mixture is firm and golden brown.

Remove and cool slightly.

In a heavy saucepan, heat the sugar and water until boiling and the sugar is dissolved. Add the lemon slices and continue cooking over medium heat until the syrup has reduced to a thick glaze. Remove the lemon slices and let drip briefly. Arrange the candied lemon slices in a circle along the outer edge of the tart next to the crust. Let cool and serve. FRESH ASPARAGUS TART (6 to 8 servings) FOR THE CRUST: 1 1/2 cups all-purpose flour 1/2 teaspoon salt 1/2 cup (1 stick) chilled unsalted butter, cut into tablespoon-size pieces 1 egg, lightly beaten 3 to 4 tablespoons cold water FOR THE FILLING: 1 pound fresh asparagus 3 large eggs, lightly beaten 1 1/2 cups whipping cream 1 teaspoon salt 1/2 teaspoon freshly ground white pepper 1/2 teaspoon freshly grated nutmeg 1 cup grated swiss or gruyere cheese

In a food processor fitted with a steel blade, place the flour and salt. Turn the machine on and off a few times to combine. Add the butter and continue pulsing, turning the machine on and off to cut the butter into the flour, until the mixture resembles cornmeal. Lightly beat the egg with the water and slowly add to the flour mixture through the feed tube, while the machine is running. Process until the mixture forms little beads. Put the dough onto a counter and gather into a ball, using your hands. Wrap up in plastic and chill 30 minutes. Roll out to a 10- to 12-inch circle and line an 8- or 9-inch quiche pan with the dough. Leave a 1-inch overhang and trim off the excess. Crimp the edges decoratively and chill until firm.

Break the asparagus naturally towards the end of the stalk where it gets tough and cut the top section to 1 1/2-inch lengths. Cook the asparagus sections for 4 to 5 minutes in boiling water. Remove and refresh in cold water. Drain again. Lightly beat the eggs, cream, salt, pepper and nutmeg. Set aside.

Line the tart crust with parchment paper or aluminum foil and fill with beans or rice. Bake the tart at 425 degrees for about 10 minutes, or until the edges are golden. Remove the beans. Bake another 5 minutes. Reduce the oven temperature to 350 degrees. Remove the tart crust.

Arrange the asparagus pieces on the crust, then sprinkle them with the grated cheese. Carefully pour the egg mixture on top and bake about 30 to 40 minutes, or until the filling is puffed, firm and golden brown. Remove, and let cool slightly. Serve.